Device For Producing Fuel Gas

ABSTRACT

The present invention relates to a device for producing fuel gases from carbon-containing material, in particular a wood gas generator, comprising a degassing zone and an oxidation zone, as well as a method for producing a fuel gas having a high calorific value by simple means, for which the degassing zone consists of a degassing chamber containing a molten metal, preferably tin, wherein the degassing chamber is designed to operate with air excluded.

The present invention relates to a device for producing fuel gases from carbon-containing material, in particular a wood gas generator, comprising a degassing zone and an oxidation zone. The present invention further relates to a method for producing fuel gases from carbon-containing material.

A wood gas generator is a device which makes it possible to obtain a combustible gas from wood by means of dry distillation. This combustible gas is used inter alia for operating motor vehicles or combined heat and power plants and is designated as wood gas. The formation of wood gas by heating wood is based on complex chemical reactions such as, for example, homogeneous and heterogeneous gasification reactions and pyrolysis processes, and a corresponding process had already been developed by Georges Imbert in 1920. The complexity of the chemical reactions is attributable not least to the fact that the wood material consists of a plurality of chemical species having various possible reactions. These species include, in particular, cellulose, lignin, hemicellulose, accessory wood components such as fats, starch, sugar, protein, phenols, wax, pectins, tannins, sterine, resins, terpene and minerals. The wood gas is produced from this diverse mixture by breaking down the fuel structure, its main components being carbon dioxide, carbon monoxide, methane, ethene, hydrogen and gaseous water. The wood is usually heated to about 700-800° C. with oxygen excluded, wherein 100 kg of wood gives about 35 m³ of wood gas per hour and leaves a residue of about 25 kg of charcoal. Also produced are about 4 kg of high-molecular aromatic hydrocarbons (tars) and about 40 kg of wood vinegar (a mixture of water, tar constituents such as phenols, creosote as well as acetic acid, methanol, acetone, propionic acid, methyl acetate and other substances). The removal of the tars from the wood gas, or better the prevention of their formation, is of major importance for the everyday suitability of such a wood gasifier. This is primarily because at temperatures below about 200° C., tars condense out and thus form thick coatings in pipes or wood gas motors.

Wood gasifiers so far comprise fixed-bed, fluidised-bed or entrained-bed gasifiers. Known from EP 1 323 810 A1 is a wood gasifier which transfers wood which has been pre-loosened in an input hopper, into a tube gasifier by means of a conveying section, moisture being removed from the wood on the conveying section to the tube gasifier. The tube gasifier heated to about 1000° C. consists of two concentric, interconnected tubes, wherein the gasification process takes place primarily in the inner tube, at the end whereof the partially degassed wood is transferred into the outer tube where it is finally ashed. This document further teaches to purify the wood gas produced from tar components in an industrial coke screw in the counterflow and to supply the purified wood gas to a combined heat and power plant. According to this document, the contaminated industrial coke should be supplied to the tube gasifier alternately with wood. Disadvantages of this prior art are the complex valve control thereby required, the expensive purification and the relatively low gross calorific value of the resulting wood gas.

It is known from DE 198 30 765 A1 to hold the wood gas for a certain time at a certain temperature so that the long-chain or cyclic high-molecular hydrocarbons (tars) contained in the wood gas are thermally cracked. This document furthermore proposes separating the steps of oxidising the fuel and degassing the fuel. The wood is degassed by heating on optionally catalytically active heat exchanger surfaces without diluting with oxidising agents such as air, whereby the heat exchanger surfaces are heated by the combustion of the degassed wood and wherein the weak gas thus produced is fed for thermal purification in a spiral line through the oxidation zone. A disadvantage with this invention is the constructively complex separation of degassing and oxidation zone with the sealing problems which arise at every transition of the combustible material.

Finally it is known from DE 694 04 861 T2 to inject carbon-containing material into a metal melt by means of a lance, to then react the carbon dissolved in the metal with blown-in oxygen to give carbon monoxide and to use the CO-containing gas thus produced as fuel gas. Complex apparatus is required for this method and the fuel gas produced has a low gross calorific value.

It is therefore the object of the present invention to provide a simple device which provides wood gas having a high calorific value.

This object is achieved whereby the degassing zone consists of a degassing chamber containing a molten metal, preferably tin, wherein the degassing chamber is designed to operate with exclusion of air. As a result of the advantageous combination of features of the invention, a fuel gas having a high calorific value is produced in a simple manner. The high calorific value is primarily attributable to the fact that dilution of the fuel gas with air containing about 79 vol. % of nitrogen inert gas is avoided. Since a liquid metal, preferably tin, is used as the temperature transfer agent, its high thermal capacity can be used to advantage. The liquid state of aggregation of the tin is achieved between 231.9° C. and 2602° C. A broad temperature range is thus available according to the invention which makes it possible to thermally control the degassing reactions depending on the combustible material. In addition, metallic tin is also non-poisonous per se even in fairly large quantities. The poison effect of simple tin compounds and salts is low. The few toxic organic tin compounds such as trialklyl tin, in particular TBT are either not formed or are only formed to an extremely small extent.

In one embodiment of the invention it is provided that the degassing chamber contains metal chips and/or catalysts, preferably in the metal bath. In this case, the metal chips are selected from those metals which remain in the solid state of aggregation at the operating temperature of the molten metal bath. The chips are used for better mixing of the melt and at the same time function as boiling stones at which the product fuel gases collect and from which they can ascend in bubble form.

If there is provided a conveying screw penetrating into the degassing chamber which is preferably configured as thermally insulated in its zones located outside the degassing chamber, continuous operation of the device according to the invention is advantageously made possible in a simple manner. The insulation prevents excessively high heat transfer. At the same time, it allows pre-drying of the combustible material in the area of the conveyor screw in front of the degassing chamber.

If the conveyor screw is configured to be sieve-like in the area of the metal bath, the combustible material can come in contact easily and completely with the metal melt. The combustible material is thus guided through the metal bath so that continuous removal takes place. No enrichment of the degassed combustible material takes place in the degassing chamber.

The amount of heat required to achieve and maintain the necessary operating temperature is provided according to the invention in that the oxidation zone consists of an oxidation chamber which at least partially encloses the degassing chamber. The degassed combustible material is combusted in the oxidation chamber whilst air is supplied and the released amount of heat is passed via the walls of the degassing chamber into the metal melt.

It is very advantageously provided according to the invention that the device comprises fuel gas purification, wherein this preferably takes place by solid-phase adsorption, for example, on activated charcoal. However, it is also feasible according to the invention to thermally treat the product fuel gas such that the tars formed decompose, in particular with the assistance of catalysts.

The process object of the invention is achieved whereby this comprises the steps: pre-drying the combustible material, degassing the combustible material in a metal bath whilst excluding air, oxidizing the degassed combustible material as well as purifying the fuel gas, preferably thermally. The process stage of degassing in a metal bath whilst excluding air very advantageously allows the formation of a fuel gas having a high calorific value since dilution of the combustible gases with oxygen or inert nitrogen is omitted.

The invention is explained in detail with reference to an exemplary embodiment shown in the single FIGURE, wherein this explanation should only be understood as exemplary and not as restrictive.

FIG. 1 shows a device according to the invention. This consists of a driven conveyor screw 1, a combustible material supply 2, a degassing chamber 3 containing a tin melt 4 and a gas space 5 as well as a combustible material removal unit 6 and an oxidation chamber 7. The combustible material is removed via the conveyor screw 1 from a storage container not shown here which can be arranged, for example, as a bulk material hopper above the conveyor screw 1. A combustible material comminuting device can also be located upstream of the bulk material hopper according to the invention if this should be necessary. The combustible material in suitable size and quantity, for example, wood chips, briquettes or the like is removed continuously from the storage container by means of the conveyor screw 1 and transported to the degassing chamber 3. At least in the area of the conveyor screw 1 adjacent to the degassing chamber 3, said conveyor screw exhibits thermal insulation, which is not shown, so that the heat transferred by the degassing chamber 3 can be used for pre-drying the wood pieces. At the same time, solidification of the tin on the walls of the conveyor screws is prevented. The combustible material is dry when it enters into the degassing chamber 3 so that explosive vaporisation and reactions of the tin with released water or its decomposition products are avoided. In the interior of the degassing chamber 3, the conveyor screw 1 is designed as sieve-like so that the liquid tin can come in contact with the wood pieces. Solid metal chips contained in the tin bath ensure good mixing and form surfaces on which the product fuel gases can collect and ascend in bubble form. The released fuel gas collects in the gas space 5 from where it can be removed actively or passively. This gas space 5 is completely filled with fuel gas and contains no air fractions. This ensures a fuel gas having a high calorific value. The conveyor screw 1 penetrates the degassing chamber 3 and ends in a combustible material removal system 6. This is also thermally insulated to prevent any solidification of the tin located therein. The fuel gas removal system 6 ends in an oxidation chamber 7 in which the degassed combustible material is oxidised. Tin possibly entrained with the degassed wood pieces can be retained in the oxidation chamber 7. The oxidation chamber 7 surrounds the degassing chamber so that the heat released via the walls of the degassing chamber 3 keeps the tin melt 4 at operating temperature. In this case, the oxidation chamber 7 also heats a section of the conveyor screw 1 so that the risk of solidification of the liquid tin is further reduced. However, it is also feasible that the oxidation chamber 7 is disposed underneath the degassing chamber 3 or penetrates this, in particular through the flue or flues. In this case, the heat is supplied particularly homogeneously to the degassing chamber 3. The fuel gas is removed from the gas space 5 and passed through a purification device which is not shown. This purification device can be an activated charcoal packing or it can consist of a thermal after-treatment. According to the invention a used activated charcoal packing can also be used as additional fuel for the oxidation chamber 7. Thus, there is no accumulation of waste for disposal.

REFERENCE LIST

-   1. Conveyor screw -   2. Combustible material supply -   3. Degassing chamber -   4. Metal melt -   5. Gas space -   6. Combustible material removal system -   7. Oxidation chamber 

1. A device for producing fuel gases from carbon-containing material, in particular a wood gas generator, comprising a degassing zone and an oxidation zone, characterised in that the degassing zone consists of a degassing chamber containing a molten metal, preferably tin, wherein the degassing chamber is designed to operate with air excluded and wherein a conveying device is provided for conveying the carbon-containing material through the metal bath.
 2. The device according to claim 1, characterised in that the degassing chamber contains metal chips and/or catalysts, preferably in the metal bath.
 3. The device according to claim 1, characterised in that a conveyor screw which penetrates the degassing chamber is provided, said conveyor screw preferably being designed as thermally insulated in its regions located outside the degassing chamber.
 4. The device according to claim 1, characterised in that the conveyor screw is designed as sieve-like in the area of the metal bath.
 5. The device according to claim 1, characterised in that the oxidation zone consists of an oxidation chamber which at least partially encloses the degassing chamber.
 6. The device according to claim 1, characterised in that this comprises a fuel gas purification system.
 7. A method for producing fuel gases from carbon-containing material, in particular wood gas, preferably using a device according to claim 1 comprising the steps of pre-drying the combustible material, degassing the combustible material in a metal bath whilst excluding air, oxidising the degassed combustible material and purifying the fuel gas, preferably thermally. 